1. Field of the Invention
The present invention relates to multilayer circuit boards in which the layers are electrically connected in the thickness direction of the circuit boards with a conductor such as conductive paste, and methods for manufacturing the same.
2. Description of the Related Art
As electrical devices have become more compact and more powerful in recent years a great need has developed to inexpensively supply industrial and consumer devices with multilayer wiring circuit boards on which semiconductor chips such as LSIs can be mounted at a high density. In such multilayer wiring circuit boards it is imperative that the plurality of layers of wiring patterns formed at a fine wiring pitch are electrically connected to one another with high reliability. In order to meet these market demands, any layer IVH structure resin multilayer circuit boards employing an inner-via hole connection method that allows interlayer connection of any electrode of a multilayer printed circuit board at any wiring pattern position have been proposed (see JP H6-268345A, for example) in place of metal plate conductors in the inner walls of through holes, which conventionally was the primary method for connecting layers in multilayer circuit boards. With such substrates, conductive paste is filled into the via holes of the multilayer printed circuit board to electrically connect only necessary layers, and because inner-via holes (IVH) can be provided directly below component lands, it is possible to achieve more compact substrates and higher density mounting. Moreover, since a conductive paste is used to achieve electrical connection in the inner-via holes, stress on the via holes can be eased and an electrical connection can be achieved that is stable even with respect to dimensional changes caused by heat or impact, for example.
Among any layer IVH structure resin multilayer circuit boards, circuit boards employing a paste fill method such as that shown in FIGS. 5A to 5D have been proposed conventionally for the purpose of achieving high-density interlayer connection with high productivity by reducing the size of the inner-via holes. A conventional method for manufacturing circuit boards that employs a paste fill method is described below. First, FIG. 5A shows an interlayer 106 in which wiring layers 102 are formed on both surfaces of an electrically insulating substrate 101 and these layers are connected by conductive paste 103 or through electrolytic plating. The electrically insulating substrate 101 is fabricated by impregnating both sides of a core material 104 with a thermosetting resin 105. The core material is a glass fabric impregnated with a thermosetting resin such as an epoxy resin, and typically a glass epoxy substrate is adopted as the substrate 101. FIG. 5B shows substrates A 110, in which a core material 107 is impregnated with an uncured thermosetting resin 108 and through holes are provided and filled with a conductive paste 109, disposed on either side of the inner layer substrate 106. The substrates A 110 disposed on either side of the inner layer substrate 106 are aligned at a predetermined position and temporarily fastened with an adhesive or the like (not shown in the drawing). Moreover, wiring layers 111 serving as outer layers are disposed at either side of the substrates A 110. Next, the uncured thermosetting resin 108 and the conductive paste 109 are cured by hot pressing, and the conductive paste 109 is compressed to adhere the wiring layer of the inner substrate and the outer layer wiring layers and achieving electrical connection between them, yielding the substrate shown in FIG. 5C. Then, the outer wiring layers are patterned by photolithography, resulting in the four-layer circuit board 112 shown in FIG. 5D. Reference numeral 111′ denotes wiring that has been patterned.
With conventional methods for forming circuit boards such as the one described above, the conductive paste flows together with the resin during hot pressing if the extent to which the uncured thermosetting resin is cured is low (gel time, flowability of the resin, minimum viscosity value in the curing process, for example). As a result sufficient compression cannot be attained, and this causes connection defects. Alternatively, a high degree of curing leads to insufficient adherence with the inner layer substrate and causes whitening due to the resin not sufficiently flowing in between the wiring pattern of the inner layer, and defects result in the moisture absorption reflow test.